[boards] stm32f4_discovery typo in description

typo at line 25: SPI1 SKL -> SPI1 SCL
added info line 15: UART2 -> UART2 (Spektrum input)

closes #552
Update README for recommended gcc-arm-embedded toolchain
[apogee] fix baro mpl3115 init on apogee
[conf] move some airframes to obsolete